Supply generator for an oscillating circuit, particularly for an induction cooking hob

ABSTRACT

A supply generator for an oscillating circuit, includes an inductance (L) and a resonant capacitor (C 3 , C 4 ), for operation at a fixed frequency and also includes at least one pair of transistors (I 2 , I 2 ), operated on a variable cyclic regime for modifying the power. The generator includes a first diode (D 5 ) between a first transistor (I 2 ) and the supply for the generator and a second diode (D 4 ) between the junction point of the inductance (L) and the resonant capacitor (C 3 , C 4 ) and the junction point of the first transistor (I 2 ) and the first diode (D 5 ). The invention is of particular use for supply of the cooking rings on an induction cooking hob.

The present invention relates to a supply generator for an oscillatorycircuit.

It also relates to a set of such generators and to an induction cookinghob comprising a plurality of generators of the invention.

The present invention is aimed generally at induction heating systems,in particular cooking hobs comprising a plurality of induction cookingrings supplied by respective generators.

These induction cooking hobs necessitate the generation in the containeror material to be heated of a current at a high frequency, of the orderof 20 to 50 kHz.

In the conventional way, this current is created by a magnetic fieldproduced by an inductor coupled to a power generator.

That power generator is generally a resonant generator, as shown in FIG.1.

That power generator is supplied with power from the electrical mainssupply at a rectified and filtered supply voltage E.

Thus each cooking ring F, comprising an inductor and a resistive load Rconsisting in particular of the container to be heated, is associatedwith resonant capacitors C₃, C₄ to form a circuit resonating at anangular frequency ω such that L(C₃+C₄)ω²=1.

The greater the combination of the chopping frequency and the generatorpower, the higher the resonance.

In induction systems, this is the case in particular when the choppingfrequency is at least 20 kHz and the power of the generator is of theorder of 3 kW.

The use of these resonant generators enables a maximum power to betransmitted to an inductive load at the resonant frequency of the supplysystem.

To prevent overheating of the semiconductors, it is possible to operatethe semiconductors of these power generators with zero switching losses.

Accordingly, in the conventional way, a soft switching mode in whichswitching occurs at the zero voltage crossing of the transistors I₁, I₂is obtained by providing the transistors I₁, I₂ with diodes D₁, D₂ andcapacitors C₁, C₂ in the usual way.

To preserve these soft switching modes, the generator power is generallyadjusted by adjusting the operating frequency around the resonantfrequency.

Power modulation by varying the operating frequency of the generator inthis way has many drawbacks, however.

In particular, the frequency range in which the generator operatingfrequency must be varied is relatively wide if the modulated power isrequired to vary in a wide range (for example in a ratio from 1 to 10).

Furthermore, if a plurality of resonant generators are operating inparallel, it is impossible to synchronize them if it is required toretain the possibility of independent power modulation.

Intermodulation noise is then generated between the generators operatingclose together at different frequencies.

One example of the above type of soft switching resonant generator isdisclosed in the document FR 2 792 157 in particular.

That document describes a solution in which a plurality of inductors maybe controlled by the same voltage and at the same frequency but with aduty cycle that may be adjusted using the pulse width modulation (PWM)technique well known in the art.

However, in the document FR 2 792 157, this mode of operationnecessitates the use of particular structures introducing the concept ofa master generator and slave generators whose operation is linked to theoperation of the master generator.

This type of structure is not very suitable for a set of inductioncooking rings in which each of the rings must operate independently,without a master and one or more slaves being defined.

An object of the present invention is to eliminate the drawbacks citedabove and to propose a supply generator for an oscillatory circuitallowing power modulation with a high power ratio at a fixed frequency.

To this end, the present invention is aimed firstly at a supplygenerator for an oscillatory circuit comprising an inductor and aresonant capacitor adapted to operate at a fixed frequency andcomprising at least one pair of transistors controlled at a variableduty cycle to modify the power.

According to the invention, the generator comprises a first diodebetween a first transistor of the pair and the supply of the generatorand a second diode between the connection point of the inductor and theresonant capacitor and the connection point of the first transistor andthe first diode.

Thanks to this particular arrangement, the operating phase of thegenerator in which the second diode conducts is relatively short.

This operating phase, corresponding to linear operation of thegenerator, is therefore very short in relation to the resonant operationof the generator, with the result that the latter's output power may bemaximized.

According to a preferred feature of the invention, the transistors areassociated with diodes and capacitors adapted to operate the generatorin a soft switching mode.

There is obtained in this way a supply generator operating at a fixedfrequency, at resonance in order to obtain maximum power in an inductiveload, and in the zero voltage switch (ZVS) soft switching mode in whichswitching occurs at zero voltage and at the nominal current.

This switching mode prevents excessive heating in the semiconductorsconstituting the power generator.

The present invention is also aimed at a set of supply generatorsaccording to the invention said generators being synchronized infrequency and controlled at different duty cycles.

Finally, the present invention is further aimed at an induction cookinghob comprising a plurality of inductors adapted to constitute one ormore cooking rings of said hob.

According to the invention, each inductor is associated with a supplygenerator in accordance with the invention, said generators beingsynchronized in frequency and adapted to be controlled independently ofeach other with a variable duty cycle.

Other features and advantages of the invention will become furtherapparent in the course of the following description.

In the appended drawings, which are given by way of non-limitingexample:

FIG. 1 is an electrical circuit diagram of a prior art supply generatordescribed hereinabove;

FIG. 2 is an electrical circuit diagram of a first embodiment of a powergenerator of the invention;

FIGS. 3, 4 and 5 are curves showing for different duty cycles the valuesof the voltages and the currents at various points of the FIG. 2electrical circuit;

FIG. 6 is an electrical circuit diagram of a second embodiment of asupply generator of the invention;

FIG. 7 is an electrical circuit diagram of a third embodiment of asupply generator of the invention; and

FIG. 8 is a block diagram of a set of supply generators of theinvention.

An electrical circuit of a first embodiment of a supply generator of theinvention is described first with reference to FIG. 2.

That generator includes two transistors I₁, I₂ in a half-bridgeconfiguration and supplied at a voltage E corresponding to the rectifiedand filtered voltage of the mains electrical power supply.

In the conventional way, these transistors I₁, I₂ are associated withdiodes D₁, D₂ and capacitors C₁, C₂ in a configuration allowingswitching in the zero voltage switching (ZVS) mode, which is a softswitching mode in which switching occurs at the zero crossing of thevoltage.

The oscillatory circuit supplied by the transistors I₁, I₂ consists ofan inductor L and resonant capacitors C₃, C₄.

This type of resonant generator transmits maximum power to inductiveloads of the L, R type such as are found in induction cooking rings, inwhich the load consists of an inductor and a container to be heated.

For example, L may have a value of the order of 50 μH and the resonantcapacitors C₃, C₄ may have a value of 680 nF.

According to the invention, a first diode D₅ is connected in series withone of the transistors of the half-bridge, here, by way of non-limitingexample, the transistor I₂.

This first diode D₅ is therefore connected between the transistor I₂ andthe supply voltage E of the generator.

A second diode D₄ is connected in parallel with a resonant capacitor C₄.

This second diode D₄ is therefore connected between the connectionbetween the inductor L and the resonant capacitor C₄ and the connectionbetween the transistor I₂ and the first diode D₅.

The diodes D₄, D₅ are connected so that the cathode of the second diodeD₄ is connected to the cathode of the first diode D₅.

Of course, an equivalent circuit could be obtained by connecting a diodein series with the other transistor I₁ of the half-bridge and a diodeacross the other resonant capacitor C₃.

The operation of a generator of the above kind controlled by controlmeans that are not shown is described next with reference to FIGS. 3, 4and 5.

Those figures show in continuous line the voltage as a function of timeat the point A of the FIG. 2 circuit, i.e. the voltage across thetransistors I₁, I₂.

The dashed line curve shows the current I_(L) flowing in the inductiveload F and the chain-dotted curve shows the voltage at the point B ofthe circuit, i.e. across the resonant capacitors C₃, C₄.

The voltage at the point A is a supply voltage at a fixed frequency,with the result that the period T of repetition of the signals isidentical in the three curves of FIGS. 3 to 5.

The period T_(on) is the time for which the transistor I₂ connected inseries with the first diode D₅ conducts.

The power delivered can therefore be varied by modifying the duty cycleδ corresponding to the ratio of the time T_(on) to the signal repetitionperiod T.

This duty cycle δ can vary from 0.5 (see FIG. 4), at which the power isat a maximum, to a value δmax (see FIG. 5) at which the power is at aminimum.

This value δmax may typically be from 0.8 to 0.9.

Thus the power is modulated by modulating the period T_(on), i.e. thetime for which the transistor I₂ conducts, and keeping the period of thesignals T constant.

Five distinct phases, numbered 1 to 5 in the figures, can therefore bedistinguished over each period T of operation:

Phase 1

The transistor I₁ conducts. The current I_(L) in the inductive loaddecreases and the resonant capacitors C₃, C₄ are discharged in resonantmode.

Phase 2

The control circuit then turns off the transistor I₁. The current I_(L)then charges the capacitors C₁, C₂ until the diode D₂ conducts, thevoltage across the transistors I₁, I₂ increasing slowly during switchingby the ZVS soft switching circuit.

During this phase, the resonant mode formed by the current I_(L) and theresonant capacitors C₃, C₄ continues.

Phase 3

The diode D₂ conducts and then the transistor I₂ also conducts. Theresonant capacitors C₃, C₄ are discharged in resonant mode with theresult that the voltage at the point B rises to a value sufficient tocause conduction in the second diode D₄.

Phase 4

The diode D₄ conducts, with the result that the current I_(L) no longerflows in the resonant capacitors C₃, C₄. The current I_(L) is dischargedslowly into the short circuit consisting of the second diode D₄ and thetransistor I₂, which continues to conduct.

This discharge occurs exponentially and not in resonant mode, and thevalue of the voltage at the point B remains equal to the value of thesupply voltage E.

Note that, during this phase 4, the current I_(L) decreases more slowlythan in the resonant mode, the current I_(L) decreasing with a slopeproportional to L/R.

Accordingly, at the end of this phase 4, the value of the current I_(L)remains positive, with the result that it is possible to turn off thetransistor I₂ using a soft switching mode.

Phase 5

The transistor I₂ is turned off and, in an analogous manner to phase 2,there is a slow decrease in the voltage across the transistors I₁, I₂ inthe ZVS switching mode.

The first diode D₅ is turned off and then the second diode D₄ is alsoturned off, with the result that the voltage B across the resonantcapacitors C₃, C₄ increases to a value greater than the value of thesupply voltage E.

This phase 5 then leads to a new phase 1 of a new period T.

The above operation is exactly the same regardless of the ratio δselected.

In particular, in FIG. 4, at maximum power, when δ is equal to 0.5, thecurrent I_(L) flowing in the load is very high, with the result that thepower output is at a maximum. In particular, the power delivered by thegenerator may be very close to that obtained at the resonant frequencywith a conventional circuit as shown in FIG. 1. The power reductioncaused by the quasiresonant operation of the generator is of the orderof only 25% to 30%.

Furthermore, the phase 4 during which the second diode D₄ conducts isvery short.

On the other hand, in FIG. 5, when the value of the ratio δ is at amaximum, a relatively low current I_(L) is obtained, corresponding to aminimum power delivered by the generator.

It is nevertheless seen that, even in this mode of operation, thecurrent I_(L) remains sufficiently high at the beginning of phases 2 and5 to preserve the ZVS soft switching mode and in particular remainssufficiently high to discharge the capacitors C₁, C₂ during theswitching phases.

Accordingly, this electrical circuit operates at full power in aquasiresonant mode adapted to the inductive loads L, R.

It is possible to operate the generator at a fixed frequency bymodulating the power by modifying the bandwidth.

The modulation depth, from δ=0.5 to δ=δmax, is relatively high andcorresponds to a power ratio of 1 to 7.

Furthermore, whatever the duty cycle δ selected, the soft switching modeis preserved by the low decrease in the current I_(L) in the circuit.

Of course, the present invention is not limited to the circuit exampleshown in FIG. 2.

In particular, it applies identically to the electrical circuit of FIG.6, which shows a second embodiment of the invention.

In this embodiment, a third diode D₆ and a fourth diode D₃, respectivelyanalogous to the first diode D₅ and the second diode D₄, are added tothe second branch of the half-bridge, with the result that the thirddiode D₆ is in series with the other transistor I₁.

The operation of the resonant generator therefore includes two linearphases, one when the current I_(L) is positive and the other when thecurrent I_(L) is negative.

Moreover, as shown in FIG. 7, it may be beneficial to replace thehalf-bridge by a complete bridge including four transistors Q₁, Q₂, Q₃,Q₄.

This circuit can offer particularly high performance if the voltagesused are very high, for example of the order of 3000 V, in which casethe power delivered by the generator can be as much as 300 kW to 400 kW.

Of course, although there is shown here the supply of power to a cookingring F in the form of an inductive load L, R, this type of generatorcould equally be used to supply a winding of a transformer.

Moreover, thanks to the capacitors C₁, C₂, the soft switching circuit ofthe transistors I₁, I₂ could also be eliminated provided that thesemiconductors are able to withstand hard switching.

As shown in FIG. 8, the resonant generator of the invention isparticularly well adapted to supply a plurality of cooking rings inparallel.

The generators can therefore be synchronized in frequency whilstoperating with different duty cycles (δ₁, δ₂, . . . δ_(n)), with theresult that the powers transmitted to the cooking rings may be adjustedindependently of each other.

This type of generator is well adapted to supplying a plurality ofcooking rings in the same induction cooking hob, in particular a cookinghob consisting of a large number of inductors in a matrix arrangement inthe hob.

1. Supply generator for an oscillatory circuit comprising an inductor (L) and a resonant capacitor (C₃, C₄) adapted to operate at a fixed frequency and comprising at least one pair of transistors (I₁, I₂) controlled at a variable duty cycle (δ) to modify the power, characterized in that it comprises a first diode (D₅) between a first transistor (I₂) of said pair and a rectified power supply of said generator and a second diode (D₄) between the connection point of the inductor (L) and the resonant capacitor (C₃, C₄) and the connection point of said first transistor (I₂) and said first diode (D₅).
 2. Generator according to claim 1, characterized in that said transistors (I₁, I₂) are associated with diodes (D₁, D₂) and capacitors (C₁, C₂) adapted to operate said generator in a soft switching mode.
 3. Generator according to claim 2, characterized in that it is adapted to switch at the zero crossing of the voltage.
 4. Generator according to claim 1, characterized in that it comprises a third diode (D₆) between a second transistor (I₁) of said pair and the supply of said generator and a fourth diode (D₃) between the connection point of the inductor (L) and the resonant capacitor (C₃, C₄) and the connection point of said second transistor (I₁) and said third diode (D₆).
 5. Set of supply generators each of which is a generator according to claim 1, characterized in that said generators are synchronized in frequency and controlled at different duty cycles (δ₁, δ₂, . . . δ_(n)).
 6. Induction cooking hob comprising a plurality of inductors adapted to constitute one or more cooking rings, characterized in that said inductors are associated with respective supply generators each of which is a generator according to claim 1, said generators being synchronized in frequency and adapted to be controlled independently of each other with a variable duty cycle. 